A conventional DC/DC converter generates a DC output voltage higher than the input power supply voltage, for example, by switching a current flowing into an inductor, with a switching element such as a MOS transistor, and then rectifying a reverse electromotive force generated in the inductor.
However, such a conventional DC/DC converter is formed using a single DC/DC conversion unit. Therefore, it has a disadvantage in that an element having a low threshold voltage must be used as the switching element in case an input supply voltage is rather low. In this case a high output voltage cannot be obtained since a withstand voltage of the switching element becomes low and, accordingly, the DC/DC converter cannot provide a high voltage step-up ratio. Conversely, use of a switching element having a high threshold voltage and a high withstand voltage to obtain a high output voltage brings about a problem that the conversion efficiency of the circuit becomes poor and the DC/DC converter does not start up in a stable mode.